1. Field of the Invention
This invention relates to magnetic thin film heads (TFH) for recording and reading magnetic transitions on a moving magnetic medium. It is applicable to either inductive or magnetoresistive (MR) elements.
In particular, the invention relates to a method of etching deep vias through the thick alumina overcoat in order to expose underlying bonding pads, thereby completely eliminating fabrication of the studs (or bonding posts), a method of etching the undercoat to define scribe-line grooves of streets and alleys across the wafer for sawing (or dicing) and machining of the sliders, and a method of pattern-etching the alumina undercoat, prior to the wafer fabrication, to create precise craters for recessed structures or open vias to the substrate for lightning arresters.
2. Background of the Prior Art
Magnetic TFH transducers are known in the prior art. See, e.g. U.S. Pat. Nos. 4,016,601; 4,190,872; 4,652,954; 4,791,719 for inductive devices and U.S. Pat. Nos. 4,190,871 and 4,315,291 for magnetoresistive (MR) devices.
In the operation of a typical inductive TFH device a moving magnetic storage medium is placed near the exposed pole-tips of the TFH transducer. During the read operation, the changing magnetic flux of the moving storage medium induces a changing magnetic flux upon the pole-tips and the gap between them. The magnetic flux is carried through the pole-tips and yoke core around spiralling conductor coil winding turns located between the yoke arms. The changing magnetic flux induces an electrical voltage across the conductor coil. The electrical voltage is representative of the magnetic pattern stored on the moving magnetic storage medium. During the write operation, an electrical current is caused to flow through the conductor coil. The current in the coil induces a magnetic field across the gap between pole-tips. A fringe field extends into the nearby moving magnetic storage medium, inducing (or writing) a magnetic domain (in the medium) in the same direction. Impressing current pulses of alternating polarity across the coil causes the writing of magnetic domains of alternating polarity in the storage medium.
Magnetoresistive (MR) TFH devices can only operate in the read mode. The electrical resistance of an MR element varies with its magnetization orientation. Magnetic flux from the moving magnetic storage medium induces changes in this orientation. As a result, the resistance of the MR element to a sensing electric current changes accordingly. The varying voltage signal is representative of the magnetic pattern stored on the magnetic medium.
In the manufacturing of TFH transducers for magnetic recording, a large number of devices are fabricated simultaneously in deposited and patterned layers on a ceramic wafer. When completed, the wafer is cut (or diced) and machined into individual slider transducers. The main elements of a TFH inductive transducer, roughly in the order in which they are deposited, are the (alumina) undercoat, the bottom magnetic pole, the flux gap material to provide spacing between the bottom and top magnetic pole-tips, one or more levels of electrical conductive coil winding interposed within insulation layers, the top magnetic pole, elevated studs (or posts) for connecting the coil to bonding pads (above the overcoat), a thick (alumina) overcoat, and the bonding pads.
Usually the studs are made of about 25-50 xcexcm thick copper, plated through a thick photoresist mask right after the completion of the top magnetic pole and prior to the deposition of thick alumina overcoat. For reasons of uniformity and reproducibility, the studs are designed to extend to a level about 10 xcexcm above the highest spot of the TFH device (the back yoke of the top pole). For each coil level the stud thickness must be increased by at least 6-9 xcexcm. For similar reasons, the alumina overcoat thickness must exceed the stud thickness by about 10 xcexcm, requiring an overcoat about 35-60 xcexcm thick. Following the overcoat deposition, the studs are covered by the alumina overcoat. The wafer is then lapped (ground) down to expose the copper studs. Next, (gold) bonding pads are fabricated over the alumina overcoat to contact the exposed studs. The pads are later used for bonding wires to external circuitry to the read/write channel. Fabrication of studs and bonding pads is described in U.S. Pat. No. 4,219,853, for example. It is similar in both inductive and MR devices. For TFH devices combining an inductive write element and an MR read element, one on top of the other, the stud and alumina overcoat operations have to be repeated. This may bring the total thickness of the studs to about 40-70 xcexcm, and the total alumina overcoat thickness to about 50-80 xcexcm. As the alumina deposition process is relatively slow, about 1-2 xcexcm/hour, it can easily become the bottleneck (1-3 days) in the process flow. Furthermore, alumina sputtering equipment is very costly. As a result, the alumina overcoat deposition is by far the costliest and the slowest step in TFH wafer fabrication.
The integrity and chipping resistance of the alumina overcoat are directly related to its thickness and stress. The thicker the alumina layer, the more stress it incorporates. The alumina overcoat""s stress also adversely affects the noise performance of the TFH device, since it is in direct contact with the top magnetic pole. Moreover, tall features of the device, such as the studs, impair the integrity of the deposited alumina in the local surrounding area. Sawing and machining operations subject the alumina overcoat to local stresses which often result in excessive chipping and rejects.
Very thick studs require a relatively long plating time and the thickness uniformity is quite difficult to control. To achieve the required thickness, stud plating dictates the use of a dry film photoresist which is difficult to strip and process. The usual fabrication of studs and bonding pads demands numerous steps of photomasking, deposition of seed-layers, plating, stripping, and etching. The elaborate structure of the studs and bonding pads, including coil leads, permalloy, and all the seed-layers, may involve some 10-15 individual metallic layers deposited in succession on top of each other. The probability of bonding failures due to inadequate adhesion between any two of these metallic layers is directly proportional to the number of these layers. Thus reliability of the bond strength is adversely affected by the large number of the metallic layers in the structure. The poor integrity of the alumina overcoat in the locales surrounding the studs further degrades the bond strength and often impairs the fabrication of bonding pads due to xe2x80x9cloopsxe2x80x9d of collapsed alumina around the studs. The latter appear following the lapping of the alumina overcoat to expose the studs, and are due to the poor integrity of the alumina surrounding the studs.
Copper residues and contamination, originating from copper seed-layers used for the coil and stud platings, may cause shorts and impair the corrosion resistance of the TFH device. Copper coil seed-layer residues are often found to be responsible for intensified corrosion susceptibility of exposed pole-tips in the air bearing surface (ABS). This poor corrosion resistance is due to galvanic action between the two dissimilar metals (Cu and Nixe2x80x94Fe) contacting at the corners (for wings) of the pole-tips. Similarly, the copper seed-layer of the studs is often found to degrade coil-to-core resistance due to full or partial shorts. The copper coil itself is sometimes subject to capricious variations of its electrical resistance due to corrosion and oxidation of the copper turns during elevated temperature curing cycles of the photoresist insulation layers.
The present invention provides an economical method for fast and precise patterning of thick sputter-deposited alumina layers by etching through appropriate metallic masks. It further provides improvements in product design and performance which avoid the above-described shortcomings of the prior art. In particular, the precise patterning of thick alumina layers by etching enables the fabrication of studless TFH devices, scribe-line grooves for dicing, recessed craters for TFH devices, and lightning arresters.
Elimination of the studs simplifies the wafer fabrication in a very substantial way. It curtails about 20-30% of the total wafer processing steps and reduces the alumina overcoat thickness by about 30-50%. It thus produces the substantial benefits of reducing processing costs, decreasing device noise, and improving throughput, consistency, bonding strength, and corrosion resistance. Alumina chipping is usually a major contributor to yield loss. Etching scribe-line grooves facilitates significant reduction of alumina chipping during slider machining and later in tests and service. It thus significantly improves total device yields. Pattern-etching precise craters in the alumina undercoat facilitates the fabrication of TFH devices with recessed structures or features. These can improve device planarity and alleviate problems associated with tall features of multi-level coil designs. They can also improve the symmetry of pole-tips configuration. Similarly, etching vias through the alumina undercoat layer enables the construction of lightning arresters by electrically connecting the magnetic core to the substrate, which will be grounded in the process of packaging the slider. The lightning arresters provide grounding to prevent arcing between the magnetic core and rotating disk.
In accordance with this invention, the patterning of thick sputter-deposited alumina layers is accomplished by, first, depositing compatible metallic layers on the alumina, coating the metallic layers with photoresist, patterning the photoresist by photolithography, etching the metallic layers (in sequence) through the photoresist mask, stripping the photoresist, wet chemical pattern-etching the thick alumina layer through the metallic mask, and removing (in sequence) the metallic masking layers. A preferred etchant comprising dilute HF in water (1:2 by volume ratio) has been found to provide fast (about 1 xcexcm/min), extraordinarily uniform (better than "sgr"=1% across a wafer), and accurate etching of thick alumina layers. The etching can be conveniently performed by spray or immersion. A compatible metallic mask must be used with the dilute HF etchant since photoresist masks lift-off from the alumina surface after a very short time. A preferred metallic mask comprises a very thin (100-300 xc3x85) Cr or Ti adhesion layer covered by a gold (Au) layer, about 0.3-0.5 xcexcm thick. To avoid porosity in the gold layer and for economic reasons, at least one half of the gold thickness can be plated from an high purity (99.9%) xe2x80x9csoftxe2x80x9d gold bath.
Other wet chemical etchants may also be used to pattern thick alumina layers. These may include hot phosphoric acid (H3PO4) and hot strong bases such as KOH, NaOH, Ca(OH)2, and hot organic amines.
In accordance with the invention a studless TFH device with gold coil and bonding pads, patterned and plated simultaneously, is provided. Following the alumina overcoat deposition, vias are etched through it to expose the bonding pads. This eliminates copper altogether from the TFH device, resulting in less maintenance and significantly improved reliability, corrosion resistance, and consistency of the coil resistance. Similarly, the thick alumina is pattern-etched to form scribe-line grooves which minimize alumina chipping during sawing and machining. Other improvements in a TFH device resulting from deep alumina pattern-etching include the formation of recessed craters to improve planarity and pole-tips symmetry and the formation of vias to the substrate for lightning arresters connecting the magnetic core to the substrate.
Several alternative embodiments which do not require etching the alumina layer are also disclosed. One such alternative includes forming xe2x80x9cplugsxe2x80x9d over the bonding pads prior to the deposition of the alumina overcoat and then lapping the wafer to expose the plugs, which are then removed by selective etching (or stripping). Another alternative includes forming plugs with negative-slope walls prior to the deposition of the alumina layer, depositing the alumina layer, and dissolving the plugs with an appropriate selective etchant, thereby xe2x80x9clifting-offxe2x80x9d the alumina overlying the plugs.
In an alternative embodiment, vias etched in the alumina layer are partially or wholly filled with studs and bonding pads, or with bonding pads alone, thereby eliminating the need for the tip of the wire-bonding tool to fit entirely inside the vias when making the wire bonds.
An object of this invention is to provide a method of pattern-etching through an alumina layer.
Another object is to provide a studless TFH device.
A further object of this invention is to provide a simple, fast, and low cost method to eliminate many processing steps and increase throughput in the process of fabricating a TFH device.
An additional object is to significantly decrease alumina overcoat thickness to further reduce processing cost and improve reliability and throughput.
Another object is to decrease alumina overcoat stress and related chipping and device noise.
An additional object of this invention is to improve bonding strength and reliability by eliminating many metallic layers in the stud structure.
An additional object is to eliminate dry photoresist processing.
An additional object is to provide gold coil and bonding pads, fabricated simultaneously, to further delete some processing steps and to entirely eliminate copper material in the TFH device.
Another object of the invention is to improve the corrosion resistance of a TFH device.
A further object of the invention is to provide scribe-line grooves for dicing and slider machining to reduce alumina chipping and improve yields.
Another object of the invention is to provide vias through the alumina undercoat to the substrate to enable the construction of lightning arresters.
A still further object of this invention is to provide precise craters in the alumina undercoat to improve device planarity and pole-tips symmetry.